To provide a treating material and a treating method for stabilizing harmful metals in a waste, particularly the harmful metals contained in an alkaline ash discharged from an incineration furnace so as not to reelute.

A mixture of ≥5wt.% thiourea ≥10wt.% powdery silicon dioxide or aluminum silicate and 1-5wt.% copper powder based on the waste or a mixture of ≥3wt.% thiourea, ≥4wt.% powdery silicon dioxide or aluminum silicate and ≥9wt.% in total of thiourea and powdery silicon dioxide or aluminum silicate and 1-2wt.% copper powder based on the waste is kneaded, if necessary with water. The specific surface area of silicon dioxide or aluminum silicate is preferably ≥150m2/g to ≤1000m2/g.